The present invention relates to an emitter-follower circuit and, more particularly, to an emitter-follower circuit which is capable of operating with safety even when an input signal applied thereto has a wide range of direct current components.
An emitter-follower circuit is one type of transistor amplifier circuit and has a configuration of a common-collector connection in which an input signal is applied to the base and an output is taken from the emitter of the transistor. The characterizing feature of the emitter-follower circuit is that it has a high input impedance, low output impedance, and near-unity voltage gain. Because of the above feature, such emitter-follower circuit is most widely used as an impedance transformer or a buffer amplifier, to prevent loading of a preceding signal source by the low input impedance of a following stage.
In the conventional emitter-follower circuits, a selection of a proper bipolar transistor to be used therein and further a determination of the circuit constants therefor are not difficult if a direct current component included in the input signal to be inputted to the circuit is known beforehand and the value thereof does not exceed the inherent breakdown voltage of the bipolar transistor and does not vary during its operating state.
However, as is the case with an output signal from a CCD (Charge Coupled Device) image sensor, when a signal in which a direct current component included in the output signal varies considerably, e.g., from 6 (V) to 13 (V) is inputted as in input signal without having a coupling capacitor intervened, a high voltage which exceeds the breakdown voltage of the bipolar transistor is developed between the collector and the emitter thereof in response to a decrease in the base voltage. Thus, for using the circuit in such applications, the most unfavorable conditions must be taken itno account and it is necessary to select a transistor which has a high breakdown voltage property between the collector and the emitter. For increasing the breakdown voltage property between the collector and the emitter of the bipolar transistor, one method now available is to adopt a high breakdown voltage process as a transistor manufacturing process. However, generally, the operating speed of such transistors as having been manufactured by the high breakdown voltage process is low and the chip area of such transistor becomes unavoidably large. All the transistors of the same kind on the single substrate are formed similarly by the same manufacturing process. If the high breakdown voltage process is adapted, the breakdown voltages of all the bipolar transistors are increased, while the overall operating speed of the circuits composed of such transistors becomes lower and this imposes limitations to the fields in which the circits may be used and also causes a large added cost due to an increase in the chip size involved.